Liquid crystals are used primarily as dielectrics in display devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are known and are based on various effects. Such devices are, for example, cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a "twisted nematic" structure, STN cells ("super twisted nematic"), SBE cells ("super birefringence effect") and OMI cells ("optical mode interference"). Displays having a high content of information actively controlled cells, for example, TFT cells ("thin film transistor"), have in particular recently become important in addition to the passively controlled, multiplexed cells. In addition to the aforementioned cell types, which are based on the use of nematic or cholesteric liquid crystals, cell types which are based on the principle of chiral tilted smectic phases are known. Thereto there belong SSF cells (surface stabilized ferroelectric), SBF cells (short-pitch bistable ferroelectric) or DHF cells (deformed helix ferroelectric). For these cells types there are generally preferred as tilted smectic phases chiral smectic C (S.sub.C *) phases, which permit especially large response speeds.
Materials for display devices based on chiral tilted smectic phases can preferably consist of a material having a tilted smectic phase, usually a S.sub.C phase, and one or more optically active dopants. The use of such chiral dopants in ferroelectric liquid crystals primarily serves to produce or alter a twisting of the tilted smectic phase and to induce a spontaneous polarization. The chiral dopants should have an adequate thermal and photochemical stability, should have a good compatibility with the tilted smectic liquid crystals, should have a viscosity which is as low as possible and should not limit the mesophase range too severely. Further, a spontaneous polarization which is as high as possible and, depending on the cell type, a rather small twisting capacity (SSF cells) or a twisting capacity which is as high as possible (SBF, DHF cells) is desirable.
As chiral dopants there also come into consideration in principle compounds which themselves have no tilted phase or generally no mesophase. This is especially frequent in the case of dopants having a very high twisting capacity. However, such dopants usually have the disadvantage that they severely narrow the tilted phase range of post mixtures to which they are added. Accordingly, dopants have been sought which do not have this disadvantage or which have this disadvantage only to a small extent.